In producing integrated circuits, it is desirable to provide packaged integrated circuits having plastic, epoxy or resin packages which encapsulate the die and a portion of the lead frame and leads. These packages have been produced using a variety of methods, a few of which will be described here.
Conventional molding techniques take advantage of the physical characteristics of the mold compounds. For integrated circuit package molding applications, these compounds are typically thermoset compounds. These compounds consist of an epoxy novolac resin or similar material combined with a filler, such as alumina. Other materials such as accelerators, curing agents, fillers, and mold release agents are added to make the compound suitable for molding.
The transfer molding process as known in the prior art takes advantage of the viscosity characteristics of the molding compound to fill cavity molds containing the die and leadframe assemblies with the mold compound, which then cures around the die and leadframe assemblies to form a solid, coherent, package which is relatively inexpensive and durable, and a good protective encapsulation for the integrated circuit.
Transfer molding operations have three stages which correspond to the three phases of viscosity of thermoset mold compounds. First there is a preheat stage required to move the mold compound from its hard initial state to the low viscosity state. Second is a transfer stage, where the compound is low in viscosity and easily transported and directed into cavities and runners. This transfer process should be rapid and should be completed before the mold compound begins to set. Finally there is a cure stage that occurs following the transfer stage. Thermoset compounds are heat cured. Other compounds may not require heat to cure, such as thermoplastics.
Conventional transfer molding techniques also require leadframes which include dambars. FIG. 1 depicts a conventional lead frame assembly and package. In FIG. 1, lead frame 21 is depicted, and package 23 is shown cutaway from the top of the lead frame 21. The leads 25 are shown coupled together outside of the outer edge of package 23 by dambar 33. Die pad 29 is positioned to receive the integrated circuit die and support the die. Die pad support strap 31 is used to keep the die and support pad 29 planar during the die attach, bonding and molding operations.
FIG. 2 depicts in a cross sectional view a lead frame and die assembly in a prior art transfer mold cavity. Leadframe 21 is placed in between a top mold chase 41 having a top mold cavity 43 and a bottom mold chase 45 having a bottom mold cavity 47. Primary runner 49 is coupled to secondary runner 51 and to gate 53. Integrated circuit die 55 is shown positioned on the die support pad of lead frame 21.
In operation, the mold compound is typically heated to attain a low viscosity state. The mold compound is then forced by compression into the primary runner 49 from a mold pot or other mold compound source, which is not shown. Secondary runner 51 then routes the mold compound over gate 53 into the cavity formed by the top and bottom cavities 43 and 47. The mold compound is forced through the narrow gate 53 into the cavity until the cavity fills with mold compound to form the encapsulated package which will have a shape defined by the shape of the top and bottom mold cavities 43 and 47.
Since the mold cavity is closed around the leadframe 21, and the leads extend outward beyond the edge of the cavities 43 and 47, there is a space between the leads, not visible in FIG. 2, where the mold compound can be forced out through the space and travel out from the cavities between the leads of leadframe 21. The dambar, which is shown in FIG. 1 but is not visible in FIG. 2, acts as a stop and allows the mold compound to travel out a short distance, but no farther. This extra mold compound forms flash between the leads, the flash forming in the area between the edge of the package and the dambar.
Therefore, in the prior art transfer molding process that uses a mold cavity to form a package around the leadframe as shown in FIG. 2, the dambar of FIG. 1 acts as a physical stop to prevent the mold compound from flowing out from the cavity area of the mold chases and forming an interlead flash or sprue joint between adjacent leads. The dambar is required to mold the package. After the package is completed, the conventional process flow is to remove the metal dambar and the flash, using metal trim and compound deflash technology. The dambar also provides lead to lead stability during processing and lead planarity during processing. Once the package is completed and cured, the dambar is no longer needed for stability and is removed.
As the number of pins increases to above 200 pins, the pitch between leads is decreasing. Also, thinner package requirements result in a requirement for thinner leadframes. So called "fine pitch" leadframes are now required. The lead to lead pitch for a fine pitch leadframe is typically less than 0.5 millimeters. The conventional dambar approach is no longer economically satisfactory for the production of fine pitch lead frames, because the trim and flash steps are so difficult to achieve.
These thinner lead frames with decreased lead to lead pitch are costly to process. Increased accuracy and handling precision is required for each step of the process. Costs of production increase as these requirements increase. Two important production cost drivers are the dambar trim and interlead flash removal steps. To remove the dambar and flash after molding for a fine pitch leadframe will require extremely precise machining during both the trim and flash removal stages of processing. The trim stage will become extremely difficult and require increasingly accurate and expensive equipment as the lead to lead pitch continues to decline. The deflash stage is a mechanical or chemical step which requires either additional precision machining or chemical processing, which also leads to chemical waste disposal costs.
Accordingly, a need thus exists for a package encapsulation molding system which uses a leadframe without a dambar to produce packaged integrated circuits having fine pitch leadframes. Eliminating the dambar also eliminates the problems and costs of the prior art trim and deflash processing steps. The molding system should provide a high part per hour throughput rate, low raw material costs, and be simple to operate, maintain, and use molding stations that are relatively inexpensive to build. The new system should be compatible with existing single pot transfer mold presses to allow a retrofitting of existing integrated circuit assembly lines.